The immunologic characterization of the leukemic cell should facilitate investigations designed to develop an understanding of the heterogeneity and pathophysiology of the acute myeloblastic and lymphoblastic leukemias and the variants of each. The intention here is to develop highly specific antisera to leukemic related antigens utilizing absorption techniques, by immunization with purified antigens isolated from either fresh leukemic cells or leukemic cell lines and by the use of somatic cell hybridization. We plan to isolate and characterize leukemia specific, associated and differentiation antigens expressed on the leukemic cell. The identification of these unique antigens by serologic methods will be the first step in their isolation for immunization and generation of antibody for passive immunotherapy. Moreover, characterization of these antigens will be required for an understanding and definition of the mechanisms responsible for the initiation of the leukemic and/or the relationship of these antigens to normal cell surface constituents. Clearly, antibody to leukemic antigens would aid in the classification of lymphocytic and myelocytic leukemias and the ability to discriminate subclasses of leukemia from one another. Lastly, highly specific antibody should provide an important means for monitoring the response of therapy by facilitating the development of assays permitting the quantitation of residual leukemic cells and circulating antigen. It is our belief that a more intimate knowledge of the leukemic cell surface will result in the discovery and characterization of yet unidentified leukemic subsets and the development of a better understanding of the pathophysiology of the leukemic process itself.